librpc/idl: Add a parser for a FILE: format keytab

librpc/idl: Add a parser for a FILE: format keytab

This will let us examine keytab entries exported for gMSA accounts and
ensure they are the values we expect.  This will in particular help
test our KDC via the "samba-tool domain exportkeytab" as this is a thin
wrapper around the relevant code.

Additionally, we can use this to test the new client-side keytab generation
in "samba-tool domain exportkeytab" for gMSA accounts.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Jo Sutton <josutton@catalyst.net.nz>